1. Field of the Invention
The present invention relates generally to the measurement of focus variations and/or critical dimension in lithographic processes.
2. Description of the Related Art
A lithographic process generally includes the exposure of a resist in a pattern with some form of energy (e.g. light, charged particles, x-rays, etc.) to which the resist is sensitive. The resist is subsequently developed to remove selected areas of the resist thereby leaving portions of the underlying layer exposed to allow processes to be selectively carried out on the exposed portions of the underlying layer. In the manufacture of integrated circuits (ICs), at least one lithographic process is generally required for the formation of any active or passive electrical element in order to define its location and its basic dimensions.
Developments in lithography have resulted in the size of features on finished ICs now being less than 0.25 microns (μm). By way of example, some current state of the art technologies incorporate 0.18 μm line width design rules. The industry is currently developing devices implementing next generation design rules (e.g., 0.13 μm, 0.08 μm etc.) and it is fully expected that in the coming years these dimensions will be further reduced.
To achieve features of these and smaller dimensions, process layers have to be manufactured with a minimum depth of focus with very stringent critical dimension control. Depth of focus is defined herein as the maximum allowable focus variation to maintain critical dimension tolerance within the finished IC. By way of example, a permissibly depth of focus of only 0.3 μm or less will be required in many processes. This extremely small depth of focus requires optimum focus control on all exposure fields on the wafer, whether they are exposed in the center of the wafer or at its edge. The effective error budget for all elements that contribute to the focus must be kept to an absolute minimum, as the total error budget of the depth of focus is the sum of all lens and body effects on the focus.
Wafer processing within these constraints is extremely challenging. The ability to measure and quantify focus and critical dimension variations throughout the production environment is important to understanding which elements contribute to degrading the focus.
The semiconductor processing equipment industry has relied heavily on Scanning Electron Microscopes (SEMs) to measure and evaluate critical dimensions on a wafer. Unfortunately, SEM measurement is a time consuming process and requires a highly skilled operator to obtain the requisite levels of accuracy. As the complexity of lithographic equipment increases and product specifications become ever more rigorous, the volume of the SEM measurements necessary to characterize a wafer has become burdensome. Accordingly, improved methods for measuring depth of focus and/or critical dimension variations across a semiconductor wafer either in testing or in the production environment that are much faster than using an SEM and at least as accurate would be desirable.